Angiotensin Converting Enzyme (abbreviated “ACE” hereinbelow) is found mainly in lungs or vascular endothelial cells. ACE acts on angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Pro-Phe-His-Leu), which has been generated by digestion with renin, to release a dipeptide (His-Leu) from its C-terminal, thereby giving angiotensin II, which causes contraction of vascular smooth muscle and has strong hypertensive effect. This enzyme also decomposes and inactivates bradykinin, which has hypotensive effect. Such ACE produces hypertensive peptide (angiotensin II) and at the same time inactivates hypotensive peptide (bradykinin), so that it exhibits hypertensive effect. Therefore, angiotensin converting enzyme inhibitor (abbreviated as ACEI hereinbelow), which inhibits activity of ACE, has hypertension inhibitory effect.
As ACEI, there are known peptides having three to ten amino acid residues including Val-Pro-Pro (Japanese Patent No. 2782142) and a tripeptide Ile-Pro-Pro (JP-A-3-120225). There is also known a peptide having ACEI activity, which is produced by digestion of milk casein by protease produced by lactic acid bacteria, and found in dissolved state in whey of fermented milk (J. Dairy Sci. 78, 6, p 1253-1257, 1995).
Such peptides as ACEI may be taken in the form of fermented milk per se containing Val-Pro-Pro and/or Ile-Pro-Pro. However, in view of the concentration and effective dose of the peptides as ACEI in the fermented milk, it is necessary to take a considerable amount of fermented milk. Thus, development of a method for producing fermented milk or whey containing a large amount of ACEI has been demanded.
It is known that ACEI such as Val-Pro-Pro and/or Ile-Pro-Pro is highly safe and thus can be used for pharmaceuticals, functional foods, health foods, and the like. For producing Val-Pro-Pro and/or Ile-Pro-Pro, there is proposed a method including the steps of culturing lactic acid bacteria in a medium containing peptides and/or proteins that have Val-Pro-Pro and/or Ile-Pro-Pro units to prepare fermented milk, and purifying the fermented milk (Japanese Patent No. 2782153).
Conventional lactic acid fermentation, for example for production of typical fermented milk products such as yogurt, is carried out by mixing starter bacteria and a starting material by stirring to form a uniform mixture, and then fermenting the mixture under static conditions in order to make the resulting product as a whole in the form of a curd. Such static conditions are believed to be required because, when a fermentation liquid is at reduced pH due to fermentative proliferation of lactic acid bacteria, application of vibration, such as by stirring or shaking, to such fermentation liquid will cause whey off and coarse texture of the resulting fermented milk products. Further, the lactic acid bacteria for the lactic acid fermentation are facultative anaerobic, so that their growth is often inhibited by oxygen. Accordingly, it has never been intended at all to effect culturing under stirring during the period where the lactic acid fermentation under static conditions is required. In cheese production, too, the fermentation is carried out by mixing starter bacteria and a starting material by stirring to form a uniform mixture, fermenting the mixture under static conditions, and then coagulating casein by the action of rennet under static conditions, after which the reaction mixture is stirred and pressed for removing whey.
Improvement in whey recovery is required for industrial purification of whey from fermented milk followed by concentration of its active components. A variety of methods for recovering the curd fraction from fermented milk have hitherto been proposed, but effective separation of whey from fermented milk has hardly been performed to date.